Lead-free organic–inorganic azetidinium alternating metal cation bromide: [(CH2)3NH2]2AgBiBr6, a perovskite-related absorber

In the last decade, organic–inorganic hybrid halide perovskite materials have developed into a very large research area in photovoltaics and optoelectronics as promising light harvesters. Lead-free double perovskites have recently been investigated as an environmentally friendly alternative to the lead-containing compositions. However, lead-free organic–inorganic hybrid halide double perovskites have so far rarely been produced due to a certain complexity in their synthesis. A number of small molecular cations have been investigated, but compositions containing azetidinium, which is a 4-membered heterocyclic molecular ring, on the A-site have hardly been considered. This study investigates the potential of [(CH2)3NH2]2AgBiBr6 as an optical absorber in photovoltaics or optoelectronics. The use of this alternative cation changes the crystal symmetry significantly. Columns of alternating metal cation form which are separated by the organic ions. While crystal symmetry is rather different from the perovskites, the overall properties as an absorber are similar. It is thus worthwhile to further investigate alternate hybrid compositions which form into other symmetries than the perovskite base structure.

Diffraction data for the structure analysis were collected on a STOE StadiMP powder diffractometer in modified Debye-Scherrer geometry using Cu Kα1 radiation (λ = 1.54059Å).The sample was sealed in a borosilicate glass capillary with a diameter of 0.3 mm to avoid preferred orientation of the crystals.The structural model was refined using the FullProf 2K program [xx] with scattering factors as implemented there.For the Rietveld refinement, three soft distance restraints were used on the carbon atoms to refine the azetidinium cation as a pseudo rigid body, d(C-C) = 1.54(1)Å. Six additional parameters were necessary to describe the anisotropic peak widths.
X-ray diffraction measurement of the thin film was performed using the PANalytical X'pert PRO diffractometer (Cu K-α1 radiation, 1.540598 Å) equipped with PIXcel3D-Medipix3 detector using a scanning line detecting mode (0.04 rad soller slit, 1/8° fixed divergence slit, 10.8 mm mask, 9.1 mm anti-scatter slit were used on the incident beam path, and 0.04 rad soller slit was used on the diffracted beam path).Pole figure measurement was performed using Euler chi-phi xyz stage 240mm for each reflection peaks of thin films fixed onto center of the stage.Continuous scanning mode was used for every orientation of which the polar angle χ (chi) was ranged from 0° to 90° with 1° step size, and the azimuthal angle φ (phi) was scanned ranging from 0° to 360° for 1 second per 1° step.XRD analysis and crystal structure determination were performed using Highscore plus software produced by PANalytical, and pole figure analysis was performed using X'pert Texture software.

Scanning Electron Microscopy (with EDX)
Scanning electron microscopy experiment was performed by the equipment from Jeol (JSM-7500F).The samples were gold-coated by vacuum vapour deposition prior to analysis.

UV-Vis Spectroscopy
Absorbance spectra were acquired from thin film samples on glass using a Shimadzu UV2600 UV-vis spectrophotometer (Japan).
For temperature-dependent measurements, the sample is placed in a Janis ST-500-SNOUT cryostat.For setting the temperature a LakeShore Model 335 temperature controller was used.After reaching the lowest temperature, 30 minutes were waited and afterwards the measurements were made from low to high temperatures.BaSO 4 powder was used for the reference of reflection data.For bandgap estimation Tauc plot was directly extracted by % diffuse reflectance spectra of powder samples and absorption spectra of thin films through the conversion to direct transition to clarify its bandgap nature.

Fig. S5 .
Fig. S5.EDX spectra of (Az) 2 AgBiBr 6 thin film through linear EDX measurement for the element investigation of the center of the grains (This film was carbon-coated to measure SEM/EDX.).

Table . S1
Atomic coordinates of the (Az) 2 AgBiBr 6 .The azetidinium cation was simulated by two carbon sites.The occupancy factors of the two carbon sites were fixed and chosen such to represent the complete scattering power of the disordered cation (3 C,1 N, 8 H).